WP 3.1 has introduced post format, which reminded me of something I came across some time ago.
I'm running a blog focused on photography, topics including shooting tips, post-production, introduction of photographers and their works, etc.
OK, first I have created 3 categories according to topics mentioned above.
Then I thought posts about photographers and their works could be sorted into different sub categories, say landscape, portrait, etc. Yes, this could be done via sub-categories.
But maybe some one would want to filter posts by the equipments used, Nikon, Canon, Sony, etc. Using category system to achieve this would mess up the category list. So I think maybe use the custom filed to do this. The problem using this method is you can add a custom filed called "camera brand", but every time you have to input "Nikon" or "Canon" rather than choosing from a list.
Now post format function is available. I'm thinking of add "link" posts, link to articles on other sites. Of course these articles need to be categorized, but as you can see, the topics covered by "link" posts could be quite different from those posts written by myself.
So now I think a multi-dimensional category system may clear things up. I could have category system A ( shooting tips/post production/photographers&works), category system B ( Nikon/Canon/Sony/Pentax/etc ), category system C ( link/quote/status/standard/etc ) ......
This might make the category system more flexible and reasonable.